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Oak wilt, a fungal disease that can be fatal to oaks and is also found in chestnut trees, has been cropping up in the Pittsburgh area. Pockets of trees in the North Hills, South Park and Frick Park have been diagnosed, according to Sandy Feather, an agent with Penn State Cooperative Extension and a PG columnist.

The wilt is caused by the fungus Ceratocystis fagacearum, which kills the tree by clogging the vascular system, until it is unable to transport nutrients. It is spread by oak bark beetles and sap beetles. Within groups of trees, infection is also spread through the co-mingled root systems of infected trees and susceptible ones. If an infected tree is closer than 50 feet to another tree, preventive measures may be called for.

Red oaks are the most susceptible to the disease, and early symptoms include leaves drooping, curling and/or turning a dull green, then bronze. Browning is frequently evident at the leaf tips or margins and sometimes along the veins. Leaves at the end of branches begin to fall soon after symptoms become noticeable, often while still green, says Ms. Feather. Leaf discoloration and defoliation continue throughout the crown of the tree for several weeks until the tree is dead. Symptoms in white oaks are similar but advance much more slowly.

The first step in containing the disease is having it accurately diagnosed by a certified arborist. Chemical treatments for the disease are unavailable to the home gardener and can only be administered by an arborist. Trees must be treated yearly. This treatment can sometimes stop the progress of the disease if caught early enough (before 30 percent of the crown is affected) and can prevent healthy plants from becoming infected.

"The single most important way to prevent oak wilt is to not prune oaks when they are actively growing. The only safe time to prune oaks is from November through early March," says Ms. Feather.

Scott C. Simpson, district manager for Davey Tree and Lawn Care in Gibsonia, says his company has dealt with outbreaks of the disease for several years now.

"Much of what I see has been caused by tree work poorly done and at the wrong time of year," says Mr. Simpson. "As pockets of disease are identified in the Pittsburgh area, the potential of disease spread will naturally increase."

He says that pruners, saws and other tools should be sanitized after working on a tree that could be infected. "Climbing spikes should never be used in any tree of any species or time of year unless it is a removal," he adds.

Mr. Simpson says the disease is here to stay because many tree owners don't understand the threat a diseased tree poses to the area tree population. Trenches dug between trees can separate roots and slow the spread of disease. He strongly recommends against mass plantings of oaks because all could be lost to the disease.

Some material in this story came from Sandy Feather's columns. Susan Banks:
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or 412-263-1516.